The present invention relates to methods and devices for producing liquid hydrocarbon products, particularly heavier products such as waxes, from gaseous reactants in a reactor. More specifically, the invention relates to methods and devices for separating liquid hydrocarbon products produced by the Fischer-Tropsch reaction using a slurry bubble column reactor.
As an alternative or supplement to refinement of fossil fuels, it is known to react synthesis gas or syngas (usually produced by steam reforming or partial oxidation of feedstocks such as natural gas), which comprises mainly CO and H2, with a catalyst such as Fe or Co to produce a wide range of hydrocarbons. This process, known as Fischer-Tropsch synthesis, is a well-known process for conversion of synthesis gas to synthetic fuels and raw materials for the chemical industry. The process is versatile in that it may use any type of coal, natural gas, or similar carbon-containing feedstock as raw material, and similarly the product distribution may be altered as desired. The product stream from known methods and devices employing Fischer-Tropsch synthesis includes, but is not limited to, naphtha, diesel, waxes, steam, water, and alcohols.
Various devices for conducting Fischer-Tropsch synthesis are known in the art, including packed bed reactors, slurry reactors such as stirred tank slurry reactors, and slurry bubble column reactors. At present, the slurry bubble column reactor is most applicable to processes utilizing Fischer-Tropsch synthesis to produce synthetic fuels and the like on a commercial basis. The slurry bubble column reactor is advantageous in comparison to the fixed or packed bed reactor system due to improved heat transfer and mass transfer, maintenance of an isothermal temperature profile, and comparatively low capital and operating costs.
In obtaining product from a slurry bubble column reactor via Fischer-Tropsch synthesis, it is necessary to separate the product from the slurry containing catalyst in order to recycle the slurry/catalyst phase into the reactor. Advantageously, in order to maximize efficiency of such a system the recycling of slurry through the slurry bubble column reactor should assume plug flow characteristics, i.e. the slurry should pass through the length of the system at a constant velocity. Prior art systems have successfully extracted product from a slurry bubble column reactor, but at the cost of maintenance of plug-flow kinetics (thereby adversely affecting efficiency of the reactor). These prior art systems further require complicated mechanisms for separating liquid products from catalyst/slurry phases in a slurry bubble column reactor.
Thus, there is a need in the art for methods and devices for separating hydrocarbon products from a slurry bubble column reactor which simply and efficiently separate the desired liquid product from the slurry/catalyst phase, maximizing separation while minimizing slurry hold-up and catalyst losses during separation. There is further a need in the art for such methods and devices which promote and enhance plug-flow characteristics of the slurry bubble column reactor, thereby maximizing efficiency and predictability of the system.
In one aspect, the present invention provides, in a Fischer-Tropsch process for synthesizing a liquid hydrocarbon product from a gaseous reactant, a method for separating a substantially particle-free liquid hydrocarbon product from a slurry comprising a catalyst particle and a suspension liquid while substantially preventing depletion of catalyst particle from the slurry. The method comprises introducing the gaseous reactant into a reactor containing the slurry, and bubbling the gaseous reactant upwardly through the catalyst particle-containing slurry to form a reaction mixture comprising liquid and gaseous hydrocarbon product, catalyst particle-containing slurry, and unreacted gaseous reactant. The gaseous reactant may be introduced into the reactor at a flow rate of from about 1 to about 20 cm/s.
A gas distributor such as a sparger may be used to bubble the gaseous reactant, typically a synthesis gas, through the slurry. Any synthesis gas resulting from conventional processing may be utilized. Typically, the synthesis gas will comprise hydrogen and carbon monoxide in a ratio of from about 0.5 to about 3.0. Suitable catalysts for the present method are those known in the art for Fischer-Tropsch reactions, including iron-based, cobalt-based, zinc-based, ruthenium-based, any catalyst based on metals from Group 8 of the Periodic Table of the Elements, or any mixture thereof. Suitable catalyst particles will have a particle size of from about 1 to about 200 xcexcm.
The reaction mixture is then passed reactor upwardly through one or more risers to discharge into a separator chamber. Typically, the separator chamber will be placed in a spaced vertical orientation with the reactor. Gaseous hydrocarbon product and unreacted gaseous reactant from the reaction mixture separate from the liquid hydrocarbon product and catalyst-containing slurry in the separator chamber, and may be removed from the separator chamber via a port and exit pipe. As is known in the art, a system of warm and cold traps may be included downstream of the exit pipe to remove wax and light oil products from the unreacted synthesis gas.
Advantageously, heavier liquid hydrocarbon products such as waxes and catalyst particle-containing slurry may be returned from the overhead separator chamber via a gravity feed to the reactor. It will be appreciated that the driving force for this recirculating flow is the difference in density between the fluid column in the riser, containing slurry and gas, and the fluid column returning to the reactor (slurry only). The liquid hydrocarbon product and slurry are returned to the reactor through at least one downcomer containing at least one cross-flow filtration element.
Typically, the cross-flow filtration element will be a device comprising a porous tube encapsulated within a shell, located within the downcomer. It will be appreciated that any suitable filter may be employed, such as a sintered metal filter, a ceramic filter, a fiber filter, a wire mesh filter, or any other suitable filtration material. Such cross-flow filtration devices are well known in the art (Kirk-Othmer Encyclopedia of Chemical Technology, 1993, Vol. 10, pages 841-847, incorporated herein by reference). The cross-flow filtration element may comprise a metal or ceramic sinter having a pore size of from about 0.05 xcexcm to about 20 xcexcm. In another embodiment, a wire mesh filter having multiple layers of mesh with variable mesh sizes (varying from coarse to finer mesh) may be used. Typically, a range of mesh sizes from about 20 to about 200 mesh is used.
Accordingly, substantially catalyst particle-free liquid hydrocarbon products (typically waxes) of the Fischer-Tropsch synthesis reaction employed herein may be axially withdrawn from the downcomer without interference with the recirculating flow described. Typically, the liquid hydrocarbon product and catalyst particle-containing slurry are passed through the downcomer at a velocity sufficient to prevent accumulation of catalyst particle on the filtration material due to the shear force provided by the slurry flow. Typically, the liquid hydrocarbon product and catalyst particle-containing slurry are passed through the downcomer at a velocity of from about 0.5 to about 100 M/min. The downcomer extends from a bottom of the separator chamber and discharges into the reactor, thereby returning slurry and catalyst to the reactor for continued use.
In another aspect, the present invention provides, in a process for synthesizing a liquid hydrocarbon product from a gaseous reactant by a Fischer-Tropsch reaction, a method for promoting plug-flow characteristics of a bubble column reactor system by establishing a natural convection loop. The method comprises essentially the steps summarized above. As noted, a recirculating flow is established, stimulated by the differences in density between the upwardly flowing mixture of synthesis gas, catalyst, and slurry, and the downward flow of liquid hydrocarbon product, catalyst, and slurry. As described above, the liquid hydrocarbon product (wax) and catalyst-containing slurry are transported from the overhead separator chamber to the reactor through a downcomer extending from the bottom of the separator chamber.
The downcomer discharges into the interior of the reactor, typically near the bottom of the reactor. Accordingly, catalyst and slurry are returned to the reactor and discharged near the bottom thereof. It will be appreciated that this feature of the method reduces the back-mixing effect of discharging the downwardly-flowing slurry into the upwardly flowing mixture of slurry/synthesis gas. Accordingly, the plug-flow nature of the reactor system is maintained. As will be described in greater detail herein, this feature provides substantial and surprising benefits over conventional slurry bubble column reactors and methods of using them. Preferably, the downcomer discharges into the bottom of the reactor at a distance sufficient to promote the desired plug flow properties of the system, without interfering with the entry of synthesis gas into the reactor. Typically, the downcomer discharges recirculating slurry into the interior of the reactor at a distance of from about 0.01 to about 0.1 M from its bottom surface.
To establish the desired recirculating flow, the gaseous reactant may be introduced into the reactor at a superficial velocity of from about 1 to about 20 cm/s. Typically, the reaction mixture flows upwardly through the reactor at a superficial velocity of from about 3 to about 15 cm/s, and liquid hydrocarbon product/slurry are returned through the downcomer at a velocity of from about 0.5 to about 100 M/min. To assure a consistent recirculation rate, the fluid level in the separator chamber may be maintained to provide a head space of from about 0.1 to about 0.5 fraction of the reactor height.
In yet another aspect of the present invention, a slurry bubble column reactor system for synthesizing a liquid hydrocarbon product and a gaseous hydrocarbon product from a gaseous reactant by a Fischer-Tropsch reaction is provided. Such slurry bubble column reactors, which utilize a slurry of Fischer-Tropsch catalyst and suspension liquid for converting gaseous reactants such as synthesis gas, are known in the art. The reactor system comprises a reactor, a gas distributor for delivering the gaseous reactant into a bottom of the reactor, and a separator chamber for separating gaseous hydrocarbon product and unreacted gaseous reactant from the liquid hydrocarbon product. Typically, the gas distributor selected is a conventional sparger.
The separator chamber is placed in a spaced vertical orientation with the reactor, and is connected thereto by a riser extending from a top of the reactor, thereby placing the reactor in fluid communication with the separator chamber. At least one port for removing the gaseous hydrocarbon product and unreacted gaseous reactant is typically provided in the separator chamber. At least one downcomer extending from a bottom of the separator chamber discharges into the interior of the reactor. Typically, the downcomer discharges near a bottom of the reactor. Each downcomer includes a cross-flow filtration element for separating a substantially particle free liquid hydrocarbon product from a downwardly flowing mixture of liquid hydrocarbon (typically a wax) and catalyst-containing slurry.
Desirably, an overhead level controller of a design known in the art may be connected to the overhead separator chamber to maintain a constant pressure head. A let-down valve actuated by the overhead level controller may meter filtered liquid hydrocarbon product into a storage tank. Accordingly, liquid level in the overhead separation chamber may be controlled by the filtration rate, and by the rate of formation of liquid hydrocarbon product (waxes). Pressure drop across the cross-flow filtration element media may be controlled by varying the storage tank pressure, which may vary from about 0 to about 100 psig. Typically, a tank pressure of 2 psig will be used.
Other objects and applications of the present invention will become apparent to those skilled in this art from the following description wherein there is shown and described a preferred embodiment of this invention, simply by way of illustration of the modes currently best suited to carry out the invention. As it will be realized, the invention is capable of other different embodiments and its several details are capable of modification in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.